A. Field of the Invention
This invention relates to the removal of SO.sub.2 from waste gases, and to an improved scrubbing system for such removal employing an aqueous solution or slurry of lime or limestone buffered by reactants that are effective in enhancing liquid-phase mass transfer.
B. Background of the Invention
Since a typical modern coal-fired power plant may emit as much as 2,000,000 SCFM waste gas containing 2,000 ppm of SO.sub.2, there is a greate deal of interest in processes for removal of SO.sub.2 from these waste gases.
The most popular commercial process used for neutralizing SO.sub.2 is an alkali aqueous scrubbing system with limestone for lime to neutralize SO.sub.2 and produce CaSO.sub.3 :
CaCO.sub.3 +SO.sub.2 .fwdarw.CaSO.sub.3 +CO.sub.2 PA1 CaO+SO.sub.2 .fwdarw.CaSO.sub.3 PA1 SO.sub.3.sup.2- +SO.sub.2 +H.sub.2 O.fwdarw.2HSO.sub.3.sup.- PA1 2HSO.sub.3.sup.- +CaO(s).fwdarw.CaSO.sub.3 (s)+SO.sub.3.sup.2- +H.sub.2 O PA1 2HSO.sub.3.sup.- +CaCO.sub.3 (s).fwdarw.CaSO.sub.3 (s)+SO.sub.3.sup.2- +CO.sub.2 +H.sub.2 O
In addition to the CaSO.sub.3, some CaSO.sub.4 is also produced because of the reaction with oxygen in the waste gas or air. The waste gas is ordinarily cooled to about 50.degree. C. by direct contact with water in the scrubber. The CaSO.sub.3 /CaSO.sub.4 solid product is disposed of as solid waste in evaporation ponds or as landfill. Because of the solids disposal involved, these processes are classified as "throw away" scrubbing.
Three basic types of throw away scrubbing include: "simple slurry", "double-alkali", and "slurry with soluble additives". Reference is made to the drawing in which FIG. 1 illustrates a simple slurry process in which the waste gas is scrubbed with a slurry of acid neutralization products and unreacted limestone. To avoid CaSO.sub.3 and CaSO.sub.4 crystallization and scaling in the scrubber, a separate crystallizer vessel is required with adequate residence time to control supersaturation of CaSO.sub.3 and CaSO.sub.4. Some of the CaO or CaCO.sub.3 (depending upon which agent is used) dissolution must also occur in the crystallizer to avoid CaSO.sub.3 scaling in the scrubber. The slurry solids concentration is typically controlled at 10-15 weight percent by clarification of a bleed stream.
In the double-alkali process, FIG. 3, SO.sub.2 is absorbed into a clear solution of soluble alkali usually NA.sub.2 SO.sub.3 :
The solution is reacted with CaO or CaCO.sub.3 in a separate vessel to precipitate CaSO.sub.3 and regenerate SO.sub.3.sup.2- :
The CaSO.sub.3 solids are separated for disposal, and a clear solution is recycled to the scrubber. Because of this the entire recycle stream must pass through solid/liquid separation, but, in return, the problems of handling slurry in the scrubber are avoided. CaSO.sub.4 is usually not crystallized as gypsum (CaSO.sub.4 2H.sub.2 O). Rather, sulfate is removed from the system as soluble purge or as a mixture of solids with CaSO.sub.3.
Some development effort has been spent on slurry scrubbing with soluble additives (FIG. 2), which is a hybrid of simple slurry scrubbing and the double-alkali process. The process uses soluble alkalis or buffers as does the double-alkali process, and also recycles slurry to the scrubber as does the simple slurry process. Slurry scrubbing with soluble additives gives more rapid rates and larger capacities for SO.sub.2 mass transfer than does a simple slurry process, but does not require as much capacity for liquid/solid separation as the double-alkali process. It has been known since 1977 that certain additives are useful as buffers and are effective in enhancing liquid phase mass transfer. Rochelle and King, by publication in INDUSTRIAL ENGINEERING CHEMICAL FUNDAMENTALS, Vol. 16, No. 1, identified adipic acid and isothalic acid as "fully effective" for mass transfer enhancement.
It is an object of this invention to provide an effective substitute for adipic acid which would be more environmentally and conservationally acceptable than adipic acid.